Sea-run cutthroat have the most variable life histories of any of the anadromous salmonids. March is probably the peak of their spawning season, but they may spawn anywhere from December through May. Most of those currently in the rivers will have entered the smaller tributaries where they will spawn by now. This is, of course, a large generalization, I've caught bright, fresh-run cutthroat while steelheading in February. Some fish enter freshwater as early as July while some don't enter until much later, so there is considerable overlap, some having finished spawning and returned to saltwater while others have not yet left the salt. Although there is almost no evidence that they ever spend more that one year in saltwater you can almost always find a few there (they are catch-and-release in all marine areas). Since cutthroat are small stream spawners and most of the smaller streams are closed during the winter, the late summer and fall are probably the best time to look for them in the rivers. My favorite sea-run cutthroat fly is Mike Kinney's Reversed Spider in a variety of colors (black and white, oddly enough, seems to be one of the most consistent) fished on a floating line.